Have to keep adding water to overflow tank...Code read PO117 for mazda miata MX5

You have a leak in the radiator, water pump, a hose, the heater or a head gasket. Check these and see if there is any water coming out of the exhaust or in the sump.
Take off the radiator cap, fill it right up to the neck, start the engine and warm it up. See if there are any bubbles in the top of the radiator, keep it topped up so there is no room in the top tank.
If there are continuous bubbles then there is a leak in the head gasket.